The present invention relates to neutron radiation detection tubes which are electronic devices which permit the passage of electrical current between electrodes when the device is exposed to the penetrating neutron radiation. More particularly, the present invention is directed to fission chamber type neutron detectors. In such a device, a high neutron cross section fissionable material such as uranium-235 is disposed within the hermetically sealed, gas filled electronic device, so that penetrating neutrons will produce further fission with the uranium-235 to produce ionization within the gas fill of the device. Such ionization produces the electrical current between the electrodes. Such fission chamber type detectors are typically used in mid-range to high range neutron flux levels. The detector tube output is proportional to the incident neutron flux and it is read directly on a DC current indicating device.
Gamma photons which may accompany the neutron flux may be compensated for by provision of a gamma compensation chamber which is typically coaxial with the fission material containing neutron chamber of the radiation detector tube. Such gamma compensation devices are well known and a typical device is set forth in U.S. Pat. No. 2,852,694. Such gamma-compensated ionization chambers are well known and reduce the signal due to gamma flux. The compensation is achieved by providing in effect two chamber volumes, one of which is sensitive to neutrons and to gamma flux, while the other is sensitive only to gamma flux. The effective gamma sensitivity of these two volumes is made equal. The output signals from the volumes are combined with opposite polarity and the signals due to gamma flux cancel each other.
The minimum neutron flux detectable by such a gamma-compensated fission chamber is determined by the magnitude of the alpha current produces by alpha particles emitted from the fissionable material within the fission chamber. Such a gamma-compensated fission chamber will typically only be effective down to a neutron flux level of 10.sup.6 neutrons per square centimeter per second. The alpha particles ionize the gas fill in the chamber and produce a constant DC current which biases any neutron current resulting from the neutron-absorption producing-fission process within the chamber. It is desirable to be able to extend the operating range of such fission chambers to lower neutron levels. The present disclosure describes a structure in which alpha compensation can be produced as well as the gamma compensation.